Thermotropic liquid crystals (LC) have demonstrated utility in the transduction of molecular events at an interface into macroscopic responses visible with the naked eye (Brake et al., 2007; Park et al., 2006; Clare et al., 2005; Kim et al., 2005; Brake et al., 2003; Luk et al., 2003; Kim et al., 2002; Gupta et al., 1998). The orientation of LC molecules is extraordinarily sensitive to physical and chemical properties of a bounding interface, and the long-range order inherent in LC phases serves to amplify surface-induced ordering for macroscopic distances. These properties, combined with the optical anisotropy of LC molecules make them well-suited for the direct transduction and amplification of the binding of an analyte to a target at an interface into an optical output (Hoogboom et al., 2003). Unlike most current methods for the detection of biological analytes, which generally require laboratory-based analytical detectors and labeled species such as fluorophores or radioactive isotopes (Blum et al., 1991), LC-based detection may be carried out in ambient light without the need for electrical power or molecular labels. This makes LC-based detection particularly useful for detection assays performed away from central laboratory locations including point-of-care, home-based, and field-based assays.
Principles of LC-based detection rely on optical, anchoring, and elastic properties arising from molecular anisotropies and the unique liquid-crystalline phase of the LC material (de Gennes et al., 1995). The molecular anisotropy of a liquid crystalline sample creates a difference in the refractive indices of light parallel and perpendicular to the bulk molecular orientation, i.e. the LC director (Dunmur et al., 2001). This difference, known as birefringence, creates a discernable optical signal that is lost when the director orients parallel to the direction of light propagation. Molecular-scale interactions between a LC and a neighboring interface result in a preferred anchoring angle relative to the surface normal (Rasing et al., 2004). Information about the interface, in the form of surface anchoring, is transmitted as far as 100 μm into the bulk (Gupta et al., 1998) as a result of the elastic nature of the LC director field.
Coupling the structure of the interface to a bioreaction, such as molecular recognition, may cause a bulk reorientation of the LCs as the reaction proceeds, generating an optical signal. The aqueous/LC interface is particularly interesting in this regard, because the aqueous phase permits convenient molecular transport to the interface and the fluidity of the interface allows for large-scale molecular re-arrangements. Furthermore, the chemical properties of the interface can be modified in a controlled way by adsorption of a surfactant monolayer (Price et al., 2007; Lockwood et al., 2005). In the absence of surfactant, a highly tilted (nearly planar) LC orientation is observed. At sufficient surfactant coverage, the tilted anchoring at the interface reorients to a homeotropic alignment (Price et al., 2007; Brake et al., 2005). We have previously shown that long-chain n-alkanoic acids adsorbed at the aqueous/LC interface possess distinct 2D phases dependent on the surfactant chemical potential and the temperature of the interface (Price et al., 2007). These phases are also characterized by molecular packing density, tilt, and lateral organization (Kaganer et al., 1999). LC anchoring is sensitive to these structural details (Brake et al., 2003).
Deoxyribonucleic acid (DNA) has long been known to form insoluble complexes with cationic surfactants in aqueous environments (Osica et al., 1997). In some cases, the complexes form highly-ordered lamellar structures, with DNA intercalated into surfactant bilayers (Radler et al., 1997). Research into DNA-lipid complexes has been predominantly driven by expectations of their use as nonviral gene carriers in transfection applications (Radler et al., 1997; Koltover et al., 1998; Miller, 1998) and in molecular diagnostics (Sastry, et al., 2000). Despite the promise of DNA/lipid complexes, DNA interactions with charged surfaces remain poorly understood (Erokhina et al., 2007). The use of Langmuir monolayers of cationic lipids has provided one method for probing DNA at such surfaces (Sastry et al., 2000; Erokhina et al., 2007; Sukhorukov et al., 1996; Kaganer et al., 1999; Ramakrishnan et al., 2004; Symietz et al., 2004; Cardenas et al., 2005; Chen et al., 2005). Möhwald et al. have shown that DNA binding to a cationic phospholipid monolayer condenses the membrane surface (Symietz et al., 2004). Furthermore, Sastry et al. measured molecular area changes upon hybridization of an oligomer target to membrane-bound DNA probes at the air/water interface (Sastry et al., 2000; Rmamkrishnan et al., 2004) and Sukhorukov et al. showed that double-stranded DNA (dsDNA) does not denature upon adsorbing to a cationic surfactant monolayer at a similar interface (Sukhorukov et al., 1996).
To date, there exists no successful method to detect nucleic acid hybridization using liquid crystals and a cationic surfactant interface. The present invention solves this and other problems in the art.